Opera includes offsetParent's border in child's offsetLeft

[sjmiles]

html.js box model functions have to account for this by querying parent's computed style and border which is expensive, relatively speaking.

This problem was originally addressed in r9341 (#3552), but the correction is bassackward.

[sjmiles]

(In [9365]) Restrict Safari margin hack to nodes that are not position: absolute, allowing more tests to pass, refs #3515. Correct getMarginBox adjustment for Opera offsetLeft/Top, refs #3565.